Crystal growth, crystal structure and linear optical ...

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oxygen atoms. The stereoisomerism and its energetical and geometrical aspects were discussed in detail by. Hoard et al. (1968). In all known bis(nitrilotriacetato)-.
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Z. Kristallogr. 215 (2000) 65±71 # by Oldenbourg Wissenschaftsverlag, MuÈnchen

Crystal growth, crystal structure and linear optical properties of the non-centrosymmetric ammonium bis(nitrilotriacetato)zirconate and hafnate, (NH4)2[Zr{N(CH2COO)3}2] and (NH4)2[Hf{N(CH2COO)3}2] P. Held I, B. Listl I, II, E. Tillmanns II, S. Ahrweiler I, H. Hellwig I and L. BohatyÂ*, I I II

UniversitaÈt zu KoÈln, Institut fuÈr Kristallographie, ZuÈlpicher Str. 49b, D-50674 KoÈln, Germany UniversitaÈt Wien, Institut fuÈr Mineralogie und Kristallographie, Althanstr. 14, A-1090 Wien, Austria

Received May 7, 1999; accepted July 2, 1999

Abstract. Large single crystals of ammonium bis(nitrilotriacetato)zirconate, (NH4 )2 [Zr{N(CH2 COO)3 }2 ] (to be written (NH4 )2 [Zr(NTA)2 ]), were grown. Linear optical properties were measured (indices of refraction and transparency range) and the phase matching conditions for second harmonic generation process were determined. Crystal structures of the zirconate and the isomorphic hafnate were solved from single crystal X-ray diffraction data A, c ˆ 16:611(4)  A, (P31; 2 21; Z ˆ 3; Zr: a ˆ 9:582(1)  Hf: a ˆ 9:5846(5)  A, c ˆ 16:5920(8)  A) and the geometry of the complex groups [Zr(NTA)2 ]2ÿ and [Hf(NTA)2 ]2ÿ was analysed.

Introduction Zirconium(IV) forms, together with the quadridentate ligand nitrilotriacetic acid, N(CH2 COOH)3 , the water-soluble complex [Zr(NTA)2 ]2ÿ (NTA ˆ {N(CH2 COO)3 }3ÿ ) which is stable in aqueous solution throughout a wide pH range (Intorre & Martell, 1960). Therefore salts of this anionic complex can be synthesized and crystallized easily in aquatic systems as was first shown by Hoard, Silverton & Silverton (1968) for the K and Rb compounds and later by Richter (1986) for the Li, Na, Cs, Tl and NH4 salts (Richter & HaussuÈhl, 1987; HaussuÈhl & Richter, 1996). The polar symmetry of the bulky complex [Zr(NTA)2 ]2ÿ ±± which consists of non-chiral components only ±± should dominate the geometry and symmetry of the crystal structure of its salts. For this reason it is to be expected that bis(nitrilotriacetato)zirconates frequently crystallize noncentrosymmetrically. The ammonium compound (NH4 )2 [Zr(NTA)2 ] was first synthesized by Richter (1986) during her Ph.D. studies. Richter grew up to 2:5  2:5  2:0 cm3 large (but always cloudy) crystals and recognized their trigonal trapezohedral symmetry. Later HaussuÈhl & Richter (1996) reported upon elastic and thermoelastic properties and gave the coefficients of thermal expansion at room temperature. Un-

* Correspondence author (e-mail: [email protected])

til now no structural data were known; not even the space group given in the literature is correct. The aim of our work was to grow large single crystals of (NH4 )2 [Zr(NTA)2 ] of optical quality (which is the basic condition for our nonlinear optical and electro-optical investigations) and to determine the crystal structure of both the ammonium bis(nitrilotriacetato)zirconate and hafnate.

Synthesis We prepared bis(nitrilotriacetato)zirconates and hafnates using the method described by Richter (1986) which was based on information published by Intorre & Martell (1960) and Larsen & Adams (1967). As starting materials for preparation of the ammonium compounds we used MCl4 (with M ˆ Zr or Hf), NH4 HCO3 and N(CH2 COOH)3 . The proportion of the starting chemicals results from: 2 N…CH2 COOH†3 ‡ 6 NH4 HCO3 ‡ MCl4 ‡x H2 O ) …NH4 †2 ‰M…NTA†2 Š ‡ 4 NH4 Cl ‡ 6 CO2 ‡y H2 O : The desired amount of NH4 HCO3 was added slowly to a suspension of nitrilotriacetic acid in water ±± while the mixture was stirred with a magnetic stirrer ±± until all solid components dissolved (solution I). An aqueous solution of MOCl2 was prepared carefully by solving MCl4 in cold water and then solution I was added. In the resulting solution (NH4 )2 [Zr(NTA)2 ] crystallized in small well-formed crystals which were subsequently purified by recrystallization. The residue solution mainly contained NH4 Cl. (NH4 )2 [Zr(NTA)2 ] and (NH4 )2 [Hf(NTA)2 ] possess unusual thermal stability: in thermogravimetric experiments (heating rate 5  C/min) the compounds did not decompose before 300  C.

Crystal growth Large single crystals of the Zr compound were grown from aqueous solutions (3 liter crystallizing vessel) at 45  C using the standard evaporation method. However, seed crystals suspended on nylon thread always yielded cloudy

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P. Held, B. Listl, L. Bohaty et al.

a

Fig. 3. Transmission spectrum of (NH4 )2 [Zr(NTA)2 ] in the wavelength range from 250 to 2200 nm.

b Fig. 1. Clinographic projection of (NH4 )2 [Zr(NTA)2 ]. Morphological rank: (a) {103}, {011}; (b) {011}, {103}.

Fig. 4. Refractive indices of (NH4 )2 [Zr(NTA)2 ] in the wavelength range from 365 to 2000 nm.

tween 1500 and 2000 nm absorption occurs as quite discrete absorption lines (Fig. 3). As a basis for our nonlinear optical investigations, the knowledge of high quality refractive indices are necessary. Refractive indices and their dispersion in the wavelength range between 365 nm

Fig. 2. Photo of a (NH4 )2 [Zr(NTA)2 ] crystal with a size of 3  3  2 cm3 .

crystals full of defects. Their uniformly developed morphology consists of the rhombohedra {103} (dominant) and {011} (small faces only) as was stated earlier by Richter (1986) and HaussuÈhl & Richter (1996) (Fig. 1a). Using (011) slabs as seed crystals, which were placed and fixed on the bottom of the crystallizing vessel, crystals of optical quality and dimensions up to 3  3  2 cm3 were grown in a period of eight to ten weeks (growth velocity about 0.4 mm/day). They have a different morphology which contains the faces of the rhombohedron {011} only with rounded edges between them (Fig. 1b). The crystals are colourless and stable in air (Fig. 2).

Linear optical properties Absorption measurements (VARIAN, CAREY 05E equipment) using (001) crystal plates show a transparency region from 280 nm up to 1500 nm, while in the region be-

Table 1. Measured refractive indices of (NH4 )2 [Zr(NTA)2 ]. The maximum error for all data given is 5  10ÿ5 . l [mm]

no

ne

0.36502 0.40466 0.43583 0.47999 0.54607 0.58765 0.64385 0.70652 0.85211 1.01398 1.08303 1.52958 1.97002

1.64149 1.63038 1.62402 1.61722 1.61025 1.60705 1.60367 1.60080 1.59621 1.59280 1.59164 1.58389 1.57719

1.66988 1.65657 1.64898 1.64097 1.63273 1.62896 1.62502 1.62164 1.61624 1.61218 1.61079 1.60154 1.59359

Table 2. Sellmeier coefficients for nabs of (NH4 )2 [Zr(NTA)2 ]. s is the sum of squares of the residuals.

no ne

D1

D2

D3

D4

2.5333(6) 2.5950(8)

0.0177(3) 0.0208(4)

0.0258(10) 0.0127(2) 0.0277(10) 0.0155(3)

s 2:85  10ÿ7 4:19  10ÿ7

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Crystal growth, crystal structure and linear optical properties of (NH4)2[Zr(NTA)2]

from Edlen's formula (Edlen, 1966) (l in mm): nair …l† ˆ 1:000083421 ‡

0:0240603 1:5997  10ÿ4 ‡ : ÿ2 130:0 ‡ l 38:9 ‡ lÿ2

The measured data nmeas and the absolute refractive indices nabs are related by the refractive index of air by nmeas ˆ nair  nabs : We fitted the refractive indices to a Sellmeier equation of type n…l†2 ˆ D1 ‡ D2 =…l2 ÿ D3 † ÿ D4 l2 : Fig. 5. Phase-matching angle q for second-harmonic generation (SHG) (type I) in (NH4 )2 [Zr(NTA)2 ] as a function of wavelength.

and 2000 nm were determined using the prism method of minimum deviation and a high precision goniometer-spectrometer (Moeller, Wedel, Goniometer Spektrometer II). For determination of refractive indices, we have to consider the refractive index of air, which can be derived Table 3. Room temperature crystal data, data collection and refinement parameters of (NH4 )2 [Zr{N(CH2 COO)3 }2 ] and (NH4 )2 [Hf{N(CH2 COO)3 }2 ].

The measured refractive indices and the Sellmeier coefficients of the absolute data are summarized in Fig. 4, Table 1 and 2. The optical character is uniaxial positive. The refractive indices and their dispersion allow type I phase-matching in second-harmonic generation (SHG) in a wavelength region between 1092 to 1990 nm (Fig. 5). Although phase-matching is uncritical for these two wavelengths, the effective nonlinear optical coefficient is zero for this process due to the symmetry 32. Therefore uncritical phase-matching is not of interest. (NH4 )2 [Zr{N(CH2 COO)3 }2 ]

(NH4 )2 [Hf{N(CH2 COO)3 }2 ]

crystal data a [ A] c [ A] V [ A3 ] M [g molÿ1 ] Z space group size [mm] Dx [Mg mÿ3 Š m [mmÿ1 Š

9.582(1) 16.611(4) 1320.8(6) 503.53 3 P32 21 spherical, r ˆ 0:09 1.900 6.04

9.5846(5) 16.5920(8) 1320.0(1) 590.80 3 P32 21 spherical, r ˆ 0:10 2.229 5.72

data collection radiation device type scan mode standard reflections standard intervall [s] decay [%] reflections (total) reflections > 3s(I) Rint hkl limits qmax [ ]

A MoKa ; l ˆ 0:7107  Nonius MACH 3 w=2q 3 3600 ÿ2.5 6956 6157 0.0129 ÿ13/13 ÿ13/13 ÿ23/23 59.9

MoKa ; l ˆ 0:7107  A Nonius MACH 3 w=2q 3 3600 ÿ0.4 6511 5819 0.0218 ÿ13/13 ÿ13/13 ÿ23/3 60.7

absorption correction type

spherical

spherical

structure refinement programm refinement on parameter unique reflection reflections [F 2 > 4s(F 2 )] R[F 2 ] R[F 2 > 4s(F 2 )] wR[F 2 ] S (D=s)max Drmax [e Aÿ3 Š Drmin [e Aÿ3 Š w1 w2

SHELX97 F2 173 2574 2252 0.0313 0.0206 0.0477 1.137 0.000 0.26 ÿ0.24 0.019 0.47

SHELX97 F2 174 2251 2082 0.0175 0.0133 0.0277 1.086 0.000 0.28 ÿ0.24 0.010 0.00

weighting scheme: w ˆ 1=‰s2 …Fo 2 † ‡ …w1 P†2 ‡ w2 PŠ mit P ˆ …Fo 2 ‡ 2Fc 2 †=3

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Table 4. Fractional atomic coordinates and equivalent isotropic dis2 placement parameter P P (A ) of (NH4 )2 [Zr{N(CH2 COO)3 }2 ]. Ueq ˆ …1 =3 † Uij ai aj ai aj

Table 5. Fractional atomic coordinates and equivalent isotropic dis2 placement parameter P P (A ) of (NH4 )2 [Hf{N(CH2 COO)3 }2 ]. Ueq ˆ …1 =3 † Uij ai aj ai aj

Atom

x

y

z

Ueq

Atom

x

y

z

Ueq

Zr N1 C3 H31 H32 C6 O3 O6 C1 H11 H12 C4 O1 O4 C2 H21 H22 C5 O2 O5 N2 H1 H2 H3 H4

0.32541(2) 0.6538(2) 0.7187(3) 0.790(4) 0.621(3) 0.7921(2) 0.7839(2) 0.8500(2) 0.4814(2) 0.463(3) 0.450(3) 0.3810(2) 0.4468(2) 0.2452(2) 0.7483(3) 0.680(3) 0.781(3) 0.8944(2) 0.8875(2) 0.0068(2) 0.8403(3) 0.874(4) 0.732(4) 0.878(3) 0.885(4)

0.0000 0.4885(2) 0.5833(3) 0.559(3) 0.547(3) 0.7625(2) 0.8140(2) 0.8496(2) 0.3690(2) 0.315(3) 0.282(3) 0.4486(2) 0.5754(2) 0.3897(2) 0.4125(3) 0.320(3) 0.375(3) 0.5289(2) 0.6483(2) 0.5039(2) 0.7850(3) 0.790(4) 0.724(3) 0.885(4) 0.744(4)

0.6667 0.89161(9) 0.8159(1) 0.789(2) 0.774(2) 0.8252(1) 0.89527(7) 0.76657(8) 0.8818(1) 0.831(2) 0.924(1) 0.8963(1) 0.94069(7) 0.86860(9) 0.9161(1) 0.948(1) 0.868(2) 0.9640(1) 0.99500(8) 0.9730(1) 0.6014(1) 0.648(2) 0.601(2) 0.585(2) 0.574(1)

0.01640(6) 0.0229(3) 0.0342(5) 0.057(8) 0.055(8) 0.0237(4) 0.0275(3) 0.0364(4) 0.0302(5) 0.044(8) 0.033(6) 0.0244(4) 0.0271(3) 0.0348(3) 0.0303(5) 0.046(7) 0.038(7) 0.0262(4) 0.0273(3) 0.0435(4) 0.0330(4) 0.08(1) 0.055(8) 0.055(9) 0.07(1)

Hfa N1 C3 H31 H32 C6 O3 O6 C1 H11 H12 C4 O1 O4 C2 H21 H22 C5 O2 O5 N2 H1 H2 H3 H4

0.32676(1) 0.6524(2) 0.7177(3) 0.790(4) 0.622(3) 0.7917(3) 0.7844(2) 0.8502(2) 0.4803(3) 0.460(4) 0.445(5) 0.3807(3) 0.4470(2) 0.2442(2) 0.7472(3) 0.685(5) 0.773(4) 0.8936(3) 0.8851(2) 0.0061(2) 0.8419(3) 0.880(4) 0.736(5) 0.867(7) 0.882(5)

0.0000 0.4877(2) 0.5833(3) 0.545(4) 0.550(3) 0.7613(3) 0.8136(2) 0.8499(2) 0.3686(3) 0.318(3) 0.285(4) 0.4480(3) 0.5757(2) 0.3895(2) 0.4117(3) 0.333(5) 0.370(4) 0.5291(3) 0.6472(2) 0.5045(3) 0.7855(3) 0.793(5) 0.725(5) 0.882(7) 0.750(6)

0.6667 0.8919(1) 0.8163(1) 0.792(2) 0.774(1) 0.8259(1) 0.89632(7) 0.76708(8) 0.8819(1) 0.828(1) 0.928(2) 0.8967(1) 0.94078(8) 0.86919(9) 0.9160(2) 0.950(2) 0.868(2) 0.9644(1) 0.99575(7) 0.9735(1) 0.6017(1) 0.646(2) 0.600(2) 0.577(2) 0.572(2)

0.01457(5) 0.0231(4) 0.0332(5) 0.052(8) 0.038(7) 0.0246(4) 0.0261(4) 0.0357(4) 0.0298(5) 0.033(8) 0.056(9) 0.0236(4) 0.0265(3) 0.0340(4) 0.0300(5) 0.06(1) 0.038(8) 0.0255(4) 0.0276(4) 0.0432(5) 0.0337(5) 0.047(9) 0.050(8) 0.09(2) 0.07(1)

i

j

i

j

a: 9.3(4)% of the Hf atoms are substituted by Zr (s.o.f.(Hf) ˆ 0.907(4), s.o.f.(Zr) ˆ 0.093(4)).

Fig. 6. Perspective projection of the complex group [Hf(NTA)2 ]2ÿ with atom labelling sheme.

NH4 ‡ . The crystal structure of the hafnate allows for the first time a study of the hafnate complex [Hf(NTA)2 ]2ÿ and a comparison with the zirconate complex. Hoard et al. (1968) described extensively the stereochemistry of the [Zr(NTA)2 ]2ÿ complex group starting out with a crystal structure analysis of the potassium salt. Unfortunately, they did not localize all molecules of water present in the structure. In fact, our thermogravimetric measurements and structural reinvestigation of the potassium salt show that the potassium salt is not a monohydrate but a dihydrate (Held, Listl, Tillmanns & BohatyÂ, 1999). In principle the results of the analysis given by Hoard et al. (1968) are correct. However, geometrical details have to be corrected slightly. Therefore we discuss the feature of the complex anion in some detail below.

Crystal structures of (NH4 )2 [Zr(NTA)2 ] and (NH4 )2 [Hf(NTA)2 ] Experimental details of the crystal structure determination at room temperature are given in Table 3. Fractional atomic parameters of both structures are listed in Table 4 and 5.

Results and discussion As expected, the crystal structures of the zirconate and hafnate are isomorphic; all structural features of both compounds agree well even in detail. The structures consists of the complex anions bis(nitrilotriacetato)zirconate(IV)= hafnate(IV) [Zr(NTA)2 ]2ÿ =[Hf(NTA)2 ]2ÿ and the cations

Fig. 7. Idealized triangulated dodecahedron [MA4 B4 ].

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Crystal growth, crystal structure and linear optical properties of (NH4)2[Zr(NTA)2]

Zirconium (hafnium) atoms are placed in special Wykoff position 3a on a twofold rotation axis and both coordinating NTA molecules are symmetrically equivalent with respect to this axis: therefore the complex group [Zr(NTA)2 ]2ÿ ([Hf(NTA)2 ]2ÿ ) possesses a point symmetry 2. In this arrangement (Fig. 6) the zirconium (hafnium) atoms have an eight-fold coordination, being surrounded by two nitrogen atoms (N1 and N10 ) and all six hydroxyl oxygen atoms (O1, O2, O3 and O10 , O20 , O30 ) of both NTA molecules. The coordination polyhedra [ZrN2 O6 ] ([HfN2 O6 ]) is a triangulated dodecahedron in which the zirconium (hafnium) atom has the hybrid schema sp3 d 4 (Mingos & Lin Zhenyang, 1990). The idealized triangulated dodecahedron [MX8 ] possesses a maximum symmetry of 42m. The ligands X are of two non-symmetrically equivalent kinds, XA and XB (hereafter referred to as A and B) (Hoard & Silverton, 1963). We use for the idealized dodecahedron [MA4 B4 ] symbols according to Hoard & Silverton (1963) which have been generally accepted since then (Fig. 7). In general there are several possibilities to occupy the eight ligand positions A4 B4 by two nitrogen and six oxygen atoms. The stereoisomerism and its energetical and geometrical aspects were discussed in detail by Hoard et al: (1968). In all known bis(nitrilotriacetato)zirconates (potassium compound (Hoard et al., 1968), (C(NH2 )3 )2 [Zr(NTA)2 ]  H2 O (HaussuÈhl, Giester & Tillmanns, 1996), Ni[Zr(NTA)2 ]  8:5 H2 O (HaussuÈhl, Giester & Tillmanns, 1997), (CH3 NH3 )2 [Zr(NTA)2 ]  2 H2 O Table 6. Selected geometric parameters ( A,  ) of the coordination polyhedra of [ZrN2 O6 ] and [HfN2 O6 ] compared to an idealized dodecahedron model [MO8 ].

M±±N1ii M±±O1ii M±±O2ii M±±O3ii

(A) (B) (A) (B)

Fig. 8. Ortep projection (Johnson, 1996) of the coordination polyhedron [HfN2 O6 ].

(HaussuÈhl & Held, 1998)) as well as in this reported crystal structures the observed stereoisomer is the Hoard type (Aagm)2 : the nitrogen atoms are placed at the A site A1(N1) and A3(N10 ) and the oxygen atoms occupy the positions A2, B1, B3 (O2, O3, O1) and A4, B4, B2 (O20 , O30 , O10 ) respectively. As a result the symmetry elements fourfold inversion axis  4, both mirror planes m and one of the twofold rotation axis of the idealized dodecahedron are lost. The remainder of the symmetry is a twofold rotation axis running through the midpoints of two edges b (edges between B1±B4 and B2±B3) (Fig. 8). This twofold rotaZr

Hf

idealized [MO8 ]: model II

2.471(2) 2.137(1) 2.179(1) 2.123(1)

2.462(2) 2.125(1) 2.168(2) 2.114(1)

M±±O ˆ 2.20  A

a

N1±±O2

2.623(2)

2.621(2)

m

N1±±O1 O2±±O3i

2.635(2) 2.535(2)

2.628(2) 2.512(2)

N1±±O3 N1±±O1i O2±±O3 O1±±O2i

2.719(2) 2.861(2) 2.797(2) 2.835(2)

2.722(3) 2.848(2) 2.782(2) 2.810(2)

O1±±O1i O3±±O3i O1±±O3

2.906(3) 3.515(3) 2.974(2)

2.903(3) 3.475(3) 2.971(2)

b ˆ 3:28  A

Zr

Hf

I

g

b

a ˆ m ˆ 2:57  A

g ˆ 2:73  A

II

III

IV

qA ˆ € (A±±M±±A) ˆ € (N1ii ±±M±±O2ii †

68.36(5)

68.62(5)

73.7

70.4

74.6

68.8

qB ˆ € (B±±M±±B) ˆ € (O1ii ±±M±±O3iii †

147.99(5)

148.23(5)

138.92

147.0

142.8

146.2

symmetry code: (i) y; x; ÿz ‡ 2; (ii) ÿy ‡ 1; x ÿ y; z ÿ 1 =3 ; (iii) ÿx ‡ 1; ÿx ‡ y; ÿz ‡ 5 =3 ; (iv) x ‡ 1; y; z; (v) ÿx ‡ y ‡ 1; ÿx ‡ 1; z ‡ 1 =3 ; (vi) ÿy ‡ 1; x ÿ y ‡ 1; z ÿ 1 =3 ; (vii) x ÿ y ‡ 1; ÿy ‡ 2; ÿz ‡ 4 =3 ; (viii) x ÿ y ‡ 1; ÿy ‡ 1; ÿz ‡ 4 =3 . I: hard-sphere model II: `most favourable model' by Hoard & Silverton (1963, 1968) III: repulsion energy calculation by Kepert (1983) IV: calculation by Mingos & Lin (1990)

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Fig. 10. Ortep plot (Johnson, 1996) of the ammonium group NH4 ‡ in (NH4 )2 [Hf(NTA)2 ] with hydrogen bonding system. For clearness the radii of all hydrogen atoms are fixed arbitrary.

Fig. 9. Ortep projection (Johnson, 1996) of the glycinate rings in (NH4 )2 [Hf(NTA)2 ]. For clearness the radii of all hydrogen atoms are fixed arbitrary.

1

/2 [d(M±±N) ‡ d(M±±O2)]=1/2 [d(M±±O1) ‡ d(M±±O3)] ˆ 1.09 follows the theoretically predicted tendency. The most important structural aspect of the complex group [M(NTA)2 ]2ÿ is the geometry of the glycinate rings. Due to the twofold rotation symmetry of the complex group it is sufficient to analyse one half of the complex only (Fig. 9, Table 7). Hoard et al. (1968) showed that a planar glycinate ring using ideal N1±±C, C±±C and C±±O bond lengths and ideal bond angles at nitrogen and carbon atoms demands a bond length d(Zr±±N1) of about 3.02  A. However, the actual distance is about 20% shorter (d(Zr±±N1) ˆ 2:471  A; d(Hf±±N1) ˆ 2.462  A). Therefore all three glycinate rings are non-planar. A simple expression of the non-planarity gives the sum of the angles in glycinate rings and its deviation from 540 (idealized sum of angles in a planar ring) (Table 7). Because of the attraction between the hydroxyl oxygen atoms (O1, O2, O3) and zirconium (hafnium) atom, the ideal bond angle of 120 at carboxyl carbons (C4, C5, C6) are considerably distorted: e.g. € (C1±±C4±±O1) ˆ 115.6(2) , € (O1±±C4±±O4) ˆ 124.0(2) . However, the carboxyl oxygen atoms (O4, O5, O6) are not noticeably affected (Table 7).

tion axis is a symmetry element of the space group P31; 2 21. Fig. 8 shows the coordination polyhedron [HfN2 O6 ], Table 6 contains geometrical values of the coordination polyhedra [ZrN2 O6 ] and [HfN2 O6 ] (bond lengths and the characteristic angles qA ˆ € (A±±M±±A) and qB ˆ € (B±±M±±B)) and a comparison with theoretically predicted geometrical values. According to the ideal symmetry (ªmost favourable modelº by Hoard & Silverton, 1963) the dodecahedral edges a, b, g, m (Fig. 7) should have the relative lengths a ˆ m < g < b and for a dodecahedron A (mean distance Zr±±O) the [MO8 ] with d(M±±O) ˆ 2.20  lengths of a, m, g and b are given in Table 6. These distances agree surprisingly well with the average values calculated from observed distances in [ZrN2 O6 ] and [HfN2 O6 ]. The angles qA and qB differ only slightly from the predicted values of the calculation by Mingos & Lin Zhenyang (1990) and by Hoard & Silverton (1963). The theoretically expected ratio of bond lengths d(M±±A)= d(M±±B) > 1 is fulfilled and its value for d(M±±O2)= d(M±±O3) ˆ 1.026 agrees very well with the value of 1.03 of the idealized model. Even the average value

P Table 7. Selected geometric parameters ( A,  ) of the glycinate rings of the complex group [M(NTA)2 ]2ÿ . ( : sum of the angles in glycinate rings). Zr N1±±C1 C1±±C4 C4±±O1 C4±±O4 N1ii ±±M±±O1ii Mv ±±N1±±C1 N1±±C1±±C4 C1±±C4±±O1 C4±±O1±±Mv P C1±±C4±±O4 O1±±C4±±O4

Hf 1.475(3) 1.516(3) 1.285(2) 1.220(2)

69.3(1) 107.2(1) 109.7(2) 115.6(2) 126.5(1) 528.3 120.4(2) 124.0(2)

1.473(3) 1.509(3) 1.288(3) 1.226(3) 69.5(1) 107.3(1) 109.7(2) 115.9(2) 126.2(1) 528.6 120.6(2) 123.6(2)

Zr N1±±C2 C2±±C5 C5±±O2 C5±±O5iv N1ii ±±M±±O2ii Mv ±±N1±±C2 N1±±C2±±C5 C2±±C5±±O2 C5±±O2±±Mv C2±±C5±±O5iv O2±±C5±±O5iv

Hf 1.476(3) 1.509(3) 1.285(2) 1.224(2)

68.4(1) 107.9(1) 110.6(2) 115.3(2) 125.9(1) 528.1 120.1(2) 124.6(2)

1.476(3) 1.516(3) 1.285(3) 1.223(3) 68.6(1) 108.1(1) 110.3(2) 115.2(2) 126.2(1) 528.4 120.0(2) 124.7(2)

Zr N1±±C3 C3±±C6 C6±±O3 C6±±O6 N1ii ±±M±±O3ii Mv ±±N1±±C3 N1±±C3±±C6 C3±±C6±±O3 C6±±O3±±Mv C3±±C6±±O6 O3±±C6±±O6

Hf 1.493(3) 1.503(3) 1.281(2) 1.221(2)

72.1(1) 108.5(1) 115.0(2) 116.7(2) 127.4(1) 539.7 119.4(2) 123.8(2)

1.494(3) 1.493(3) 1.288(2) 1.229(3) 72.6(1) 108.1(1) 115.2(2) 117.0(2) 126.8(2) 539.6 119.8(2) 123.1(2)

symmetry code: (i) y; x; ÿz ‡ 2; (ii) ÿy ‡ 1; x ÿ y; z ÿ 1 =3 ; (iii) ÿx ‡ 1; ÿx ‡ y; ÿz ‡ 5 =3 ; (iv) x ‡ 1; y; z; (v) ÿx ‡ y ‡ 1; ÿx ‡ 1; z ‡ 1 =3 ; (vi) ÿy ‡ 1; x ÿ y ‡ 1; z ÿ 1 =3 ; (vii) x ÿ y ‡ 1; ÿy ‡ 2; ÿz ‡ 4 =3 ; (viii) x ÿ y ‡ 1; ÿy ‡ 1; ÿz ‡ 4 =3 .

71

Crystal growth, crystal structure and linear optical properties of (NH4)2[Zr(NTA)2] Table 8. Hydrogen-bonding geometry ( A,  ) involving the NH4 ‡ group. D±±H . . . A

N2±±H1 . . . O6 N2±±H2 . . . O5vi N2±±H3 . . . O6vii N2±±H4 . . . O4viii

D±±H

H...A

D. . .A

€ D±±H . . . A

Zr

Hf

Zr

Hf

Zr

Hf

Zr

Hf

0.83(3) 0.90(2) 0.88(3) 0.84(3)

0.80(3) 0.88(2) 0.92(5) 0.80(5)

2.09(3) 2.29(3) 2.22(3) 2.16(3)

2.14(3) 2.33(3) 2..23(6) 2.17(4)

2.804(3) 3.111(3) 3.094(3) 2.867(3)

2.806(3) 3.130(3) 3.091(3) 2.871(3)

143(3) 152(2) 168(3) 142(3)

140(2) 151(2) 154(3) 147(4)

symmetry code: (i) y; x; ÿz ‡ 2; (ii) ÿy ‡ 1; x ÿ y; z ÿ 1 =3 ; (iii) ÿx ‡ 1; ÿx ‡ y; ÿz ‡ 5 =3 ; (iv) x ‡ 1; y; z; (v) ÿx ‡ y ‡ 1; ÿx ‡ 1; z ‡ 1 =3 ; (vi) ÿy ‡ 1; x ÿ y ‡ 1; z ÿ 1 =3 ; (vii) x ÿ y ‡ 1; ÿy ‡ 2; ÿz ‡ 4 =3 ; (viii) x ÿ y ‡ 1; ÿy ‡ 1; ÿz ‡ 4 =3 .

The ammonium cation NH4 ‡ is surrounded by seven oxygen atoms which belong to three different [M(NTA)2 ] groups. Five of them are carboxyl oxygen atoms (O4, O40 , O5, O6, O60 ) and two are hydroxyl oxygen atoms (O2, O3) (Fig. 10). The NH4 group is fixed via hydrogen bonds to four carboxyl oxygen atoms (O4, O5, O6, O60 ) (Table 8). Assuming a NH‡ 4 cation with a spherical space demand and an ionic radius of 1.48  A, a distance d(N2±±O) of about 2.85  A is to be expected. The observed distances (Table 8) range between 2.804  A and 3.130  A. Therefore it seems that the hydrogen bonds do not shorten the distance d(N2±±O) significantly. However, these bonds preclude the rotation of the NH4 groups. This effect gives a considerable contribution to the thermal stability of the crystal structures: in both compounds no structural phase transition was observed to the point of decomposition at about 310  C; this is rather a rare example in the family of ammonium salts. By comparing the structure of the zirconate complex with the hafnate complex, Zr has been seen to have slightly longer (Dd  0:01  A) coordination distances to N and O than has Hf (Table 6). All bond lengths d(Zr±±N) and d(Zr±±O) of [ZrN2 O6 ] in the Zr compound are about 0.010  A to 0.015  A longer than the corresponding distances in the Hf compound. These values are five to ten times larger than the standard deviations of the experimental data. Therefore this information seems to be significant. The same is true for pairs of isostructural compounds K2 [M(NTA)2 ]  2 H2 O, Rb2 [M(NTA)2 ]  2 H2 O and Cs2 [M(NTA)2 ]  H2 O (Held, Listl, Tillmanns & BohatyÂ, 1999). The crystal structure analysis of (NH4 )2 [Hf(NTA)2 ] yielded a Zr content of about 9% with respect to the Hfposition (Table 5), which is more than twice the Zr content in our starting Hf source (HfCl4 ). This signalizes that the distribution coefficient for Zr between solution and crystal, k ˆ c…crystal†=c…solution†, should be larger than 1. We observed a similar effect of zirconium enrichment during the first stage of crystallization from aqueous solutions in the case of Zr containing alkali metal bis(nitrilotriacetato) hafnates.1 1 Supplementary Material: Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 135315 and CCDC 135316. Copies of available material can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: ‡44-(0) 12 23-33 60 33 or e-mail: [email protected]). The list of Fo =Fc -data is available from the author up to one year after the publication has appeared.

Acknowledgment. The authors are gratefully indebted to Dr. C. Wickleder, Institute of Inorganic Chemistry, University of Cologne, for assistance in the measurement of the transmission spectrum. Financial support for this study was provided by International Centre of Diffraction Data ICDD under grant 90-03 to E. Tillmanns.

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